This invention relates to voltage-controlled crystal oscillators (VCXO), and more particularly to voltage-controlled capacitors for VCXO""s.
Many digital systems rely on precise clocks to sequence through states and process data. Crystal oscillators are often used to generate the precise clocks needed by these systems. A voltage-controlled crystal oscillator (VCXO) is often used.
FIG. 1 is a diagram of a prior-art crystal oscillator. Crystal 14 oscillates at a fundamental frequency when a gain stage provides gain to start the crystal oscillating and then to maintain the oscillation. Crystal 14 is coupled between nodes X1 and X2, and is usually connected to other components such as inverter 10 by pins on an integrated circuit (IC). Inverter 10 inverts node X1 and drives node X2, acting as the gain stage. Feedback resistor 12 acts as a DC bias that biases inverter 10 in its gain region.
Capacitors 16, 18 provide a load capacitance to ground for nodes X1, X2. The value of capacitors 16, 18 can alter the frequency of oscillation of crystal 14. Any given crystal has a manufacturer-specified load capacitance that causes the crystal to oscillate at exactly the specified frequency. Larger capacitive loads on nodes X1, X2 slow down the oscillation, while smaller capacitive loads on nodes X1, X2 accelerate the oscillation. Values of 20-40 pF are common for capacitors 16, 18.
Sometimes it is desirable to alter or tune the frequency of oscillation. A control voltage can be applied to adjust the oscillator frequency. A VCXO consists of a conventional crystal oscillator that is tuned by an externally applied voltage. The tuning voltage changes the frequency of oscillation. A crystal oscillation frequency is determined primarily by physical dimensions and process. Most VCXO""s have a very high quality factor (Q), which implies that the frequency of oscillation is very tightly controlled and varies only a few hundred parts per million (ppm). The narrow pull range is an important advantage in telecom systems, HDTV, and other application areas.
FIG. 2 shows a voltage-controlled crystal oscillator (VCXO). Feedback resistor 12 and inverter 10 are coupled across nodes X1, X2 of crystal 14.
The capacitance of nodes X1, X2 is set by DC isolation capacitors 15, 17. These are much larger capacitors than in FIG. 1, being perhaps 200 pF. Larger capacitor values are needed since two capacitors 15, 22 are in series. The equivalent capacitance of two series capacitors 15, 22 is the reciprocal of the sum of the reciprocals of the two capacitances, Cseries =1/(1/C15+1/C22).
A control voltage Vc is applied to the back of DC isolation capacitor 15 through resistor 26, and to the back of DC isolation capacitor 17 through resistor 28. This control voltage Vc can range from ground to the power-supply voltage VDD. The DC voltage of nodes X1, X2 is about VDD/2, set by feedback resistor 12, which connects the input and output of inverter 10. DC offset capacitors 15, 17 isolate nodes X1, X2 from the control voltage Vc.
Diode varactor capacitors 22, 24 are also coupled to the backsides of DC isolation capacitors 15, 17. Control voltage Vc can change the capacitance value of diode varactor capacitors 22, 24. The equivalent series capacitance of capacitors 15, 22 or 17, 24 also changes, changing the capacitive load on nodes X1. X2. This varies the loading capacitance and oscillation frequency of the VCXO. Unfortunately, diode varactors are not available in standard CMOS process, but are expensive components produced by a special manufacturing process.
The control voltage Vc can be increased, causing the diode varactor capacitance of capacitors 22, 24 to decrease, and the crystal oscillation frequency to increase. Otherwise when Vc decreases, the diode capacitance of capacitors 22, 24 increases, so the crystal oscillation frequency decreases. Thus the frequency of oscillation can be adjusted by adjusting the control voltage Vc.
Additional inverters (not shown) can be driven by node X2. These additional inverters can then drive clock buffers that drive clocks to various destinations.
While such a voltage-controlled oscillator is useful, the size of DC isolation capacitors 15, 17 is quite large. The diode varactor""s capacitive-coupling ratio is reduced by parasitic capacitances of the large DC isolation capacitors, reducing circuit efficiency. The large size of the DC isolation capacitors make it more expensive to integrate them onto the same silicon chip as inverter 10 and other system components. Instead, external DC isolation capacitors and diode varactors are sometimes used, either on-chip of off-chip.
What is desired is a voltage-controlled crystal oscillator without the large DC isolation capacitors. Smaller capacitors that can be integrated onto the same system chip as the oscillator""s feedback inverter are desired. A VCXO with integrated capacitors that adjust the oscillator frequency in response to a control voltage is desired.
A voltage-controlled crystal oscillator (VCXO) has a crystal that is coupled between a first node and a second node. The crystal oscillates at an oscillation frequency when connected to a gain inverter. The gain inverter has an input driven by the first node. The gain inverter drives the second node to induce oscillation by the crystal. A feedback resistor is coupled between the first and second nodes.
A first metaloxide-semiconductor (MOS) capacitor has a source/drain coupled to the first node and a gate coupled to a first gate node. The first MOS capacitor has a capacitance that varies with a first input voltage of the first gate node. A second MOS capacitor has a source/drain coupled to the second node and a gate coupled to a second gate node. The second MOS capacitor has a capacitance that varies with a second input voltage of the second gate node.
The first and second gate nodes are connected together and driven by an input voltage. The oscillation frequency and capacitances of the first and second MOS capacitors vary when the input voltage varies. Thus variations in the input voltage vary the oscillation frequency by adjusting capacitances of the first and second MOS capacitors.